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Wales - Enterprising venture

Highways - The showpiece of a new privately financed road in Wales is a steel decked cable stay bridge.

'A spectacular cable stay bridge will form a landmark structure and symbol of regeneration for the Sirhowy valley in south Wales when it opens next year.

The £34M Sirhowy Enterprise Way project comprises 4.3km of new highway, with two cross-valley connections and a five-span viaduct. It will provide a bypass for Blackwood town centre, and open up access to Oakdale Business Park, under development on the site of the disused Blackwood colliery. The new road will allow all 69ha of the site to be developed rather than just the 28ha accessible by the existing road network.

Client Caerphilly Borough Council expects Oakdale to generate or sustain over 10,000 jobs, and called for a landmark structure in its project specification.

A 30-year design, build, finance and operate contract has been granted to Sirhowy Enterprise Way, a consortium of Laing and Costain. Costain is acting as main contractor with Arup as designer.

Bestriding the valley with its 87m pylon and its road deck 30m above the valley floor, the cable-stayed Pont Dewi Sant forms the centrepiece of the project. The single pylon arrangement was chosen partly to minimise work on the Sirhowy flood plain. 'The logistics of construction are quite difficult, ' says Costain structures agent Barry Woodman. 'It's a very narrow site with steep access down into the valley.'

Practicality made steel the obvious material for the deck, says Woodman. 'Because it's 30m above the valley, the cost of the falsework for a concrete bridge would have been prohibitive. Even a post tensioned or balanced cantilever structure would inevitably have been slower to construct, and heavier, ' he says.

The deck exemplifies the trend toward composite construction, with 1.5m deep steel plate girders as edge beams, linked by cross members to form a ladder deck. Omnia precast units will form permanent shuttering for an insitu concrete deck. 'We chose that form for its combination of cost-effectiveness and buildability, ' says Arup project design manager Richard Sanders.

Construction is well under way, with the pylon 85% finished.

All the backspan steelwork is in place and work on the deck is about to start, with the west abutment finished and work on the east abutment under way.

Unusually, the deck is being constructed on temporary trestles while the pylon is still going up: the conventional method is to finish the pylon first and then install deck and stays together. 'The bridge is the critical structure on the contract, ' says Woodman. 'This way of going about it is slightly more expensive but gives you a quicker programme.' Trestles for the backspan will be placed at 24m centres and are capable of carrying the steel and the dead weight of the deck. To reduce the cost of the temporary works, though, trestles for the main span are spaced at 48m and are designed to carry the weight of the steel only. This means casting of the insitu deck will have to wait until the stays are in place.

Total length of the bridge is 227m with a main span of 121m.

The original outline design had a short backspan, but Arup adjusted the position of the pylon to bring the two spans closer in length, which removed the need for a balancing anchorage pier on the backspan. The 18.6m wide deck, which carries a single carriageway plus cycle track in each direction, is 18.6m between the stays.

The pylon itself is concrete, with a steel anchorage chamber for the stays cast into place at the top.

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